DE112016000983B4 - Fuel injection control device for a direct injection engine - Google Patents

Abstract

A fuel injection control device for a direct injection engine, the control device comprising:an engine block including a combustion chamber defined by a cylinder head cover portion, a cylinder block provided for a cylinder, and a piston reciprocating inside the cylinder; anda fuel injection control unit including a fuel injection valve provided within the combustion chamber for injecting a liquid fuel and configured to inject the fuel into the combustion chamber at a predetermined timing, whereinthe fuel injection control unit is adapted to perform a main injection via a to perform a period ranging from a final stage of a compression stroke to an initial stage of a power stroke and also to perform a pilot injection over a period lasting from the intake stroke to a first half of the compression stroke, in which a smaller amount of the fuel than in the main injection, and the fuel injection control unit is also adapted to determine whether the fuel injected by the pilot injection produces a partial oxidation reaction during a second half of the compression stroke calls, and upon determining that the fuel causes the partial oxidation reaction to perform an intermediate injection between the pilot injection and the main injection, and the fuel injection control unit is adapted to perform the intermediate injection at such a timing that allows the intermediate injection Injection allows fuel injected to ignite spontaneously during or after the fuel injected by the main injection spontaneously ignites, said timing being before the partial oxidation reaction occurs during the second half of the compression stroke.

Description

TECHNICAL AREA

The present disclosure relates to a fuel injection control device for a direct injection engine.

TECHNICAL BACKGROUND

JP 5 477 246 B2 describes that auto-ignition combustion is performed in a gasoline engine whose geometric compression ratio is set to be 15 or more. This engine injects the fuel into a cylinder twice in a high load region where the amount of injected fuel increases, so that the fuel is injected separately during the intake stroke and the compression stroke. The engine allows a fuel-air mixture in a cavity provided at the top of a piston to self-ignite in the vicinity of compression top dead center. The engine then allows the fuel/air mixture to ignite and burn outside the cavity with the intense heat generated by combustion during the power stroke. This enables self-ignition combustion to be performed without increasing combustion noise even in a high load region where the amount of injected fuel increases.

JP 2013- 057 266 A discloses a direct injection engine having an outwardly opening fuel injector located at the central axis of a cylinder, which injects the fuel in the form of a hollow cone. In this direct injection engine, fuel is injected into the cylinder during the latter half of the compression stroke, thereby forming an air-fuel mixture layer in a combustion chamber and a gas layer surrounding the air-fuel mixture layer. the inside JP 2013- 057 266 A The disclosed engine reduces cooling loss by causing the surrounding gas layer to serve as a heat insulating layer during combustion of the fuel/air mixture.

A similar fuel injection control device is also from the reference DE 11 2011 101 619 B4 known, with a main injection taking place there near top dead center and a preceding injection in the compression stroke before the main injection, with the control device timing said preceding injection as a pilot injection in such a way that part of the fuel spray reaches the outside of a piston recess, with a pilot injection also being delayed in time is made after the pilot injection to suppress ignition of the fuel injected by the pilot injection.

Other similar fuel injection control devices are also known from the references DE 10 2015 000 590 A1 , which is post-released, as well as DE 10 2011 117 712 A1 known.

DEPICTION

TECHNICAL PROBLEM

Forming a fuel/air mixture layer and a gas layer surrounding the fuel/air mixture layer in the manner described in JP 2013- 057 266 A disclosed combustor favors the reduction of cooling loss. However, the heavier the load applied to the engine in operation, the larger the amount of fuel injected and the harder it is to form a gas layer around the air-fuel mixture.

In this case, forming a fuel-lean gas layer by performing a small amount of pilot injection at an early stage from the intake stroke to the first half of the combustion stroke enables forming a fuel-air mixture layer closer to the central portion of the combustion chamber by performing a main injection to a period ranging from the final stage of the compression stroke at to an initial stage of the power stroke, as in JP 5 477 246 B2 discloses that the air-fuel mixture formed around the air-fuel mixture layer acts as a heat-insulating layer and lowers the combustion temperature around the wall surface of the combustion chamber while the engine is operated under high load. This favors a reduction in cooling loss.

However, the fuel sucked into the cylinder during the period from the intake stroke to the first half of the combustion stroke (i.e., the fuel injected by the pilot injection) may occur in an environment where the pressure and temperature rise gradually in the cylinder when the cylinder is the compression stroke rises, cause a partial oxidation reaction. Once the fuel drawn in by the pilot injection has caused a partial oxidation reaction, the fuel injected during the period lasting from the final stage of the compression stroke to the initial stage of the power stroke (i.e., the fuel injected by the main injection) will spontaneously ignite after it has been injected and before it has mixed well with the air (i.e. without being given sufficient ignition timing retard), thereby possibly generating smoke or lengthening the combustion period during the power stroke so as to cause an increase in CO emission.

In view of the above background, it is therefore an object of the present disclosure to reduce a drop in exhaust emission performance by making discrete injections in a high-load range in which the amount of injected fuel increases, a fuel injected by main injection that is given a sufficiently long ignition timing retard over a timing ranging from the final stage of the compression stroke to the initial stage of the power stroke.

THE SOLUTION OF THE PROBLEM

In order to at least partially solve or mitigate the above problems, the present invention provides a fuel injection control device for a direct injection engine according to claim 1. Preferred developments of the invention are the subject matter of the dependent claims. That is, the fuel injection control device includes: an engine block including a combustion chamber defined by a cylinder head cover portion, a cylinder block provided for a cylinder, and a piston reciprocating inside the cylinder; and a fuel injection control unit including a fuel injection valve provided in the combustion chamber to inject a liquid fuel and configured to inject the fuel into the combustion chamber at a predetermined timing.

The fuel injection controller performs a main injection during a period ranging from a final stage of a compression stroke to an initial stage of a power stroke, and also performs a pilot injection over a period ranging from the intake stroke to the first half of the compression stroke which a smaller amount of fuel is injected than with the main injection. Further, the fuel injection control unit determines whether the fuel injected by the pilot injection causes a partial oxidation reaction during a second half of the compression stroke, and when determining that the fuel causes the partial oxidation reaction, performs an intermediate injection between the pilot injection and the main injection. The fuel injection control unit performs the intermediate injection at such a timing that allows the fuel injected by the intermediate injection to be spontaneously ignited during or after the fuel injected by the main injection and before the partial oxidation reaction during the second half of the Compression stroke occurs to ignite spontaneously.

As used herein, "first half of the compression stroke" and "second half of the compression stroke" refer to the first and second halves, respectively, when the compression stroke is divided into two equal halves, namely a first half and a second half. Meanwhile, the “final stage of the compression stroke” herein means the final stage when the compression stroke is divided into three equal stages, namely, an initial stage, a middle stage, and the final stage. Also, herein, the “initial stage of the power stroke” means an initial stage when the power stroke is divided into three equal stages, namely, the initial stage, a middle stage, and the final stage.

According to this embodiment, the fuel injection control device performs discrete injections, which include at least a pilot injection and a main injection. This is advantageous in a high load region where the amount of fuel injected increases. The pilot injection is performed by injecting the fuel into the cylinder over a period lasting from the intake stroke to the first half of the compression stroke. Because the fuel is injected relatively early, the injected fuel diffuses within the combustion chamber. Further, the pilot injection injects a relatively small amount of fuel, and forms a relatively lean air-fuel mixture.

The main injection is performed by injecting the fuel into the cylinder for a period ranging from the last stage of the compression stroke to the initial stage of the power stroke. Since the fuel is injected late, the fuel spray concentrates toward a central portion of the combustion chamber. The main injection injects a relatively large amount of fuel. In this way, at a time when the air-fuel mixture ignites, an air-fuel mixture layer and a relatively lean gas layer surrounding the air-fuel mixture layer are formed in the combustion chamber. When the air-fuel mixture layer ignites spontaneously in this state, the surrounding gas layer acts as a heat-insulating layer, and the combustion temperature around the wall surface of the combustion chamber falls, whereby cooling loss can be reduced.

The fuel injected into the cylinder by the pilot injection is subjected to a pressure and temperature which gradually increase as the compression stroke progresses, causing a partial oxidation reaction in some cases during the second half of the compression stroke. As used herein, the "partial oxidation reaction" refers to a situation where the fuel exhibits an insufficient thermal flame response but instead causes an oxidation reaction. Although the amount of heat generated by the partial oxidation is smaller than in the case of a complete oxidation reaction, it still causes an in-cylinder temperature rise. Therefore, once the fuel injected by the pilot injection has caused the partial oxidation reaction, the fuel injected during the main injection will spontaneously ignite thereafter without being given a sufficiently long ignition timing delay.

According to the above-described embodiment, the fuel injection control unit determines whether or not a partial oxidation reaction will occur, and when the answer is YES, performs an intermediate injection between the pilot injection and the main injection. The intermediate injection is performed before a low-temperature oxidation reaction occurs. The latent heat of vaporization of the fuel injected into the cylinder by the intermediate injection lowers the temperature in the cylinder, thus substantially preventing the partial oxidation reaction from occurring. Suitably, the intermediate injection is performed at a time before the partial oxidation reaction occurs. This enables the intermediate injection function, which lowers the temperature in the vaporization latent heat cylinder, to effectively prevent the partial oxidation reaction from occurring.

Also, if the intermediate injection is performed too early, the intermediate injection could not fully perform the function of suppressing the occurrence of the partial oxidation. Further, the fuel injected by the intermediate injection could also cause the partial oxidation reaction. The intermediate injection is thus performed at such a timing that allows the fuel injected by the intermediate injection to ignite spontaneously during or after the fuel injected by the main injection spontaneously ignites. This substantially prevents the fuel injected by the intermediate injection from causing the partial oxidation reaction.

In this way, preventing the partial oxidation reaction from occurring during the second half of the compression stroke gives the fuel injected into the cylinder by the main injection a sufficient ignition timing delay, making it possible to prevent the generation of smoke or the increase in CO emission.

The fuel injection control unit may determine whether or not a partial oxidation reaction will occur based on an in-cylinder pressure, an in-cylinder temperature, and an in-cylinder oxygen concentration. A determination as to whether or not a partial oxidation reaction will occur can be accurately made based on these parameters. Thus, performing the intermediate injection when the partial oxidation will occur substantially prevents the partial oxidation from occurring.

The intermediate injection can inject a smaller amount of fuel than the main injection.

Since the intermediate injection starts earlier than the main injection, the temperature and pressure in the cylinder are not sufficiently high at the time of the intermediate injection. Thus, it becomes more and more likely that the fuel spray injected into the cylinder will travel longer than expected and possibly come into contact with the wall surface of the combustion chamber. This is disadvantageous when forming the gas layer surrounding the fuel/air mixture. For this reason, the amount of fuel injected by the intermediate injection is relatively small. Furthermore, reducing the fuel injected by the intermediate injection enables injection of much fuel by the main injection. This favors engine torque in the half to full load range where the total amount of fuel injected increases.

The main injection may be multi-stage injection including a plurality of fuel injections, and an interval between an injection end of the intermediate injection and an injection start of the main injection may be longer than an interval between first and second injections of the main injection.

When discrete injections are performed, the spray of fuel injected earlier may interfere with the spray of fuel injected later, potentially creating a locally overly dense fuel-air mixture. In this case, it will cause not only an increase in combustion temperature but also a decrease in exhaust emission performance. According to the above In the configuration described above, sufficiently lengthening the injection interval between the intermediate injection and the main injection substantially prevents the spray of fuel injected by the intermediate injection from interfering with the spray of fuel injected by the main injection. This advantageously helps reduce cooling loss by lowering combustion temperature and substantially preventing smoke generation by eliminating local formation of an overly dense fuel/air mixture.

ADVANTAGES OF THE INVENTION

As can be understood from the above description, the fuel injection control device for the direct injection engine performs an intermediate injection between the pilot injection and the main injection, and thus by lowering the temperature in the cylinder with the latent heat of vaporization of the injected fuel, substantially prevents that Occurrence of a partial oxidation reaction. As a result, the fuel injected by the main injection is allowed to ignite spontaneously with a sufficiently long allowance of ignition timing retardation, remarkably reducing a decrease in exhaust emission performance.

character list

• 1 FIG. 1 schematically illustrates a configuration for a direct injection engine.
• 2 12 is a cross-sectional view schematically illustrating the configuration of a combustor.
• 3 12 illustrates how the effective cross-sectional area of the orifice orifice of an outwardly opening fuel injector varies with stroke amount.
• 4 12 illustrates an example operating map when an engine is warm.
• 5 illustrates a fuel injection mode in a full load range.
• The drawing above 6 schematically illustrates fuel sprays formed in a combustion chamber when pilot injection is performed in the high-load range, and the lower diagram of FIG 6 shows the distribution of temperatures in the combustion chamber in such a situation.
• The drawing above 7 FIG. 12 schematically illustrates fuel sprays formed in the combustion chamber at a top dead center in the full load range and the bottom diagram of FIG 7 shows the distribution of temperatures in the combustion chamber in such a situation.
• The drawing above 8th FIG. 12 schematically illustrates a fuel spray formed in the combustion chamber when main injection is completed in the high-load range, and the lower diagram of FIG 8th illustrates the temperature distribution in the combustion chamber in such a situation.
• 9 illustrates a fuel injection mode in a half to full load range.
• 10 FIG. 12 schematically illustrates a fuel/air mixture layer formed in the combustion chamber in the half to full load range.
• 11 shows how the heat generation rate changes depending on whether intermediate injection is performed or not.
• 12 shows how the in-cylinder temperature changes depending on whether the intermediate injection is performed or not.

DESCRIPTION OF EMBODIMENTS

Embodiments will now be described with reference to the accompanying drawings. The following description is only an example.

(overall configuration of the engine)

1 12 illustrates a configuration for a motor 1 according to the embodiment. Although not shown, the crankshaft 15 of the engine 1 is coupled to driving wheels via a transmission. The power of the engine 1 is transmitted to the drive wheels through a transmission 1, propelling the vehicle forward. The fuel of the vehicle 1 in this embodiment is gasoline, but may be gasoline containing bioethanol or another additive. The present disclosure is broadly applicable to any other type of premixed fuel that ignites via vaporization of a fuel that has been injected and that uses any of various types of liquid fuels.

The engine 1 includes a cylinder block 12 and a cylinder head 13 mounted on the cylinder block 12. Inside the cylinder block 12 are a plurality of cylinders 11 (of which 1 only one is shown). The engine 1 is a multi-cylinder engine. Although not shown, inside the cylinder block 12 and the cylinder blocks 13 formed a water jacket in which the cooling water flows. A piston 16 coupled to the crankshaft 15 via a connecting rod 14 is slidably fitted in each of the cylinders 11 . The piston 16 defines, together with the cylinder 11 and the cylinder head 13, a combustion chamber 17.

In this embodiment, the top portion 170 of the combustion chamber 17 (i.e., the underside of the cylinder head 13) is configured to have a lateral intake slope 171 provided with an opening 180 of an intake port 18 and having an upward gradient toward the center of the cylinder 11, and a to have an exhaust lateral slope 172 which is provided with an opening 190 of an exhaust port 19 and which has an upward gradient towards the center of the cylinder 11. The combustion chamber 17 is of the so-called “pent roof type”. The ridge of the pent roof may or may not coincide with the bore center of the cylinder 11. As also in 2 shown, the top 160 of the piston 16 is raised to form a pitched roof shape formed by rising surfaces 161, 162 each having an upward gradient towards the piston 16 on the suction side and the exhaust side, respectively, by the To be facing slope 171 of the suction side and the slope 172 of the outlet side of the ceiling portion 170 . Thus, the geometric compression ratio of the engine 1 is set to be 15 or more. The top 160 of the piston 16 also has a recessed cavity 163.

Although in 1 only one intake port 18 is shown, two intake ports 18 are actually provided for the cylinder head 13 of each cylinder 11 . The respective openings 180 of the two suction ports 18 are arranged side by side in the direction of the output shaft (that is, the crankshaft 15) of the engine on the slope 171 of the suction side of the cylinder head 13. The intake ports 18 communicate with the combustion chamber 17 through these openings 180. Likewise, for the cylinder head 13 of each cylinder 11, two exhaust ports 19 are provided. The respective openings 190 of the two exhaust ports 19 are arranged side by side in the direction of the engine's output shaft on the slope 172 of the exhaust side of the cylinder head 13 . The exhaust ports 19 communicate with the combustion chamber 17 through these openings 190.

Each of the suction ports 18 is connected to an associated suction passage 181 . A throttle valve 55 for controlling the intake flow rate is provided halfway through the intake passage 181 . Each of the outlet ports 19 is connected to an associated outlet passage 191 . At the exhaust passage 191, an exhaust gas purification system having one or more catalysts is arranged. Each of the catalysts may include a three-way catalyst. However, the catalytic converter does not have to be a three-way catalytic converter.

The cylinder head 13 is provided with an intake valve 21 arranged to isolate the intake ports 18 from the combustion chamber 17 (that is, to close the combustion chamber 17). The intake valve 21 is driven by an intake valve driving mechanism. The cylinder head 13 is also arranged with an exhaust valve 22 to be able to isolate the exhaust ports 19 from the combustion chamber 17 . The exhaust valve 22 is driven by an exhaust valve drive mechanism. The intake valve 21 and the exhaust valve 22 reciprocally move at a predetermined timing to open and close the intake port 18 and/or the exhaust port 19 respectively, thereby exchanging gases in the cylinder 11 .

Although not shown, the intake valve actuation mechanism includes an intake camshaft operatively connected to the crankshaft 15 . The intake camshaft rotates synchronously with the rotation of the crankshaft 15. Although not shown, the exhaust valve actuation mechanism includes an exhaust camshaft operatively connected to the crankshaft. The exhaust camshaft rotates synchronously with the rotation of the crankshaft 15.

In this example, the intake valve actuation mechanism includes at least one hydraulic or electrically variable valve timing (VTT) mechanism 23 capable of continuously changing the phase of the intake camshaft within a predetermined angular range. The intake valve driving mechanism may further include a VVT mechanism capable of changing the valve lift amount as well as the VVT mechanism 23 .

In this example, the exhaust valve actuation mechanism includes a hydraulic and/or electrically variable valve timing mechanism VVT 24 capable of continuously changing the phase of the exhaust camshaft within a predetermined angular range. The intake valve actuation mechanism may further include a variable valve lift mechanism capable of changing the valve lift amount as well as the VVT mechanism 24 .

The variable valve lift mechanism may also be a continuous variable valve lift mechanism (CVVL) capable of continuously changing the valve lift amount. It should be noted that the valve actuation mechanism for driving the intake valve 21 and the exhaust valve 22 may also be another type of driving mechanism, such as a hydraulic or electromagnetic driving mechanism.

As in 2 As shown on a larger scale, a fuel injection valve 6 for injecting the fuel directly into the combustion chamber 17 is mounted on the cylinder head 13 . The fuel injection valve 6 is arranged on the ridge of the pent roof, at which the slope 171 on the suction side and the slope 172 on the outlet side intersect. Also, the fuel injection valve 6 is arranged such that its injection axis S is aligned with the axis of the cylinder 11 and the tip end of its injection port faces the inside of the combustion chamber 17 . It should be noted that the injection axis S of the fuel injector 6 may or may not be aligned with the axis of the cylinder 11 .

The cavity 163 of the plunger 16 is arranged to face the fuel injection valve 6 . The fuel injection valve 6 injects the fuel into the cavity 163 .

In this example, the fuel injection valve 6 is an outwardly opening fuel injection valve. The outward-opening fuel injection valve 6 has a nozzle body 60 with an orifice 61 through which the fuel is injected, and an outward-opening valve 62 that opens and closes the orifice 61, which is in 3 is shown showing the end of its tip on a larger scale.

The nozzle body 60 is formed as a tubular member such that fuel flows through the interior of the nozzle body 60 . The nozzle opening 61 is provided at the tip end of the nozzle body 60 . The nozzle orifice 61 is tapered such that the diameter gradually increases toward the tip.

The outwardly opening opening valve 62 has a valve body 63 exposing to the outside of the valve body 60 at the tip of the nozzle body 60, and a connecting portion 64 extending from the valve body 63, extending through the nozzle body 60 and having a (not shown) piezoelectric element is connected. The valve body 63 includes a seat portion 65 which has substantially the same shape as the tapered nozzle opening 61. There is a reduced diameter portion 66 between the seat portion 65 and the connecting portion 64 of the valve body 63. As in FIG 3 As shown, the reduced diameter portion 66 has a different slope than the seat portion 65 . Concretely, the reduced-diameter portion 66 inclines from the bottom end toward the tip more gently than the seat portion 65.

As indicated by the two-dot line in 3 indicated, the nozzle opening 61 is closed, while the seat portion 65 rests against the nozzle opening 61. The piezoelectric element is deformed when a voltage is applied thereto, and thus lifts the outwardly opening valve 62 along the injection axis S. Consequently, the seat portion 65 moves as indicated by the solid line in FIG 3 indicated away from the nozzle opening 61, thus opening the nozzle opening 61 to open. When the nozzle hole 61 is opened, the fuel is injected through the nozzle hole 61 in a direction inclining with respect to the injection axis S and radially expanding from the injection axis S. Specifically, the fuel is injected in the form of a hollow cone whose central axis is defined by the injection axis S. When the voltage application to the piezoelectric element stops, the piezoelectric element returns to its original shape in which the seat portion 65 of the outwardly opening valve 62 abuts the nozzle hole 61 and the nozzle hole 61 closes again.

As the voltage applied to the piezoelectric element increases, the lift amount of the outwardly opening valve 62 from the state in which the nozzle opening 61 is closed increases. How out 3 As is clear, the greater the lift amount of the nozzle hole 61 becomes, the greater the degree of opening (that is, the effective cross-sectional area). In this case, the effective cross-sectional area is defined by the distance from the nozzle opening 61 to the seat portion 65. FIG. As the lift amount increases, the particle size of the fuel spray injected through the nozzle hole 61 into the combustion chamber 17 also increases. Conversely, as the lift amount increases, the particle size of the fuel spray injected through the nozzle hole 61 into the combustion chamber 17 also increases. Also, the fuel flows along the reduced-diameter portion 66 while passing through the nozzle hole 61 . As the lift amount increases, the reduced-diameter portion 66 is further spaced from the nozzle hole 61, and therefore the spray angle of the fuel (i.e., the taper angle of the hollow cone) becomes narrower. Meanwhile, the reduced-diameter portion 66 comes closer to the nozzle hole 61, and therefore the spray angle of the fuel (that is, the taper angle of the hollow cone) becomes wider.

Also, it is assumed that the fuel pressure is constant, the wider the effective cross-sectional area, the smaller the obtained becomes injection rate. On the other hand, the injection rate increases as the effective cross-sectional area increases. However, if the effective cross-sectional area becomes too small, the effect on the fuel of the frictional resistance caused by the wall surface of the injection hole becomes so remarkable that the injection rate becomes low. Thus, there is a lift amount at which the fuel injection rate becomes maximum. The fuel injection rate drops regardless of whether the lift amount is larger or smaller than the maximum rate of the lift amount. It should be noted that this maximum lift amount is relatively small.

As in 2 As shown, the ceiling portion 170 of the cylinder head 13 has a recess 173 recessed from the ceiling surface, and the tip end of the fuel injector 6 is received in the recess 173 . The inner surface of the recess 173 has such a slope that its inner diameter gradually increases toward the interior of the combustion chamber 17 . Locating the tip end of the fuel injection valve 6 at a position lower than the ceiling surface of the cylinder head 13 can make the gap between the top 160 of the piston 16 and the tip end of the fuel injection valve 6 as wide as possible when the piston 16 reaches top dead center while the geometric compression ratio increases. This promotes the formation of the heat-insulating gas layer around the air-fuel mixture layer as will be described later. Further, the gap between the tip end of the fuel injection valve 6 and the inner surface of the recess 173 widens enough to prevent the fuel spray injected from the fuel injection valve 6 from adhering along the ceiling surface of the cylinder head 13 due to the Coanda effect.

A fuel supply system 57 includes an electric circuit for driving the outwardly opening valve 62 and a fuel supply system that supplies the fuel to the fuel injection valve 6 . The engine controller 100 outputs an injection signal having a voltage corresponding to the lift amount to the electric circuit at a predetermined timing, and thus operates the outward opening valve 62 via the electric circuit so that a desired amount of fuel is injected into the cylinder becomes. When the injection signal is not output (that is, when the injection signal has zero voltage), the nozzle orifice 61 is closed by the valve 62 opening outward. In this way, the operation of the piezoelectric element is controlled by the injection signal from the engine controller 100. FIG. The engine controller 100 controls the operation of the piezoelectric element, thereby controlling the injection of the fuel through the nozzle opening 61 of the fuel injection valve 6 and the lift amount at the time of fuel injection. Because the piezoelectric element is easily responsive, multi-stage injections can be performed, for example, about twenty injections can be performed in 1 to 2 milliseconds. However, the means for operating the outwardly opening valve 62 is not limited to a piezoelectric element.

Although not shown, the fuel supply system is provided with a high-pressure fuel pump and a common rail. The high-pressure fuel pump pressure-feeds the fuel that has been fed from a fuel tank by a low-pressure fuel pump to the common rail, which stores the pressure-fed fuel at a predetermined fuel pressure. Then, the fuel injection valve 6 is operated (that is, the outwardly opening valve 62 is lifted) to inject the fuel stored in the common rail through the nozzle hole 61 . A fuel injection control unit includes the engine controller 100 and the fuel injection valve 6 .

As will be described later in detail, the fuel injection control unit is configured such that a (combustible) air-fuel mixture layer and a heat-insulating gas layer surrounding the air-fuel mixture layer can be formed in the combustion chamber 17 (that is, in the cavity 163 ), as schematically in 2 is shown.

This engine 1 is set up to burn the fuel/air mixture formed in the cylinder 11 by compression ignition (ie by controlled self-ignition (CAI)) virtually over the entire operating spectrum. The engine 1 includes an ignition assistance system 56 in order to assist the fuel/air mixture in igniting in a predetermined environment. Ignition assistance system 56 can be a discharge candle, for example, which is arranged facing combustion chamber 17 . In other words, applying controlled pulses of high voltage to the electrode of the discharge plug to generate an extremely short pulse discharge in the combustion chamber 17 enables current discharge to be generated in the combustion chamber, thereby generating ozone in the cylinder. The ozone supports the CAI. It should be noted that the ignition assistance system does not have to be designed as a discharge candle to generate ozone, but can also be a "spur" candle to support CAI by applying energy to the fuel/air mixture through a spark discharge.

The engine 1 further includes an EGR system 512 configured to reintroduce burned gas into the cylinder 11 . The EGR system 512 includes both an external EGR system for reintroducing the burned gas into the cylinder 11 through an EGR passage connecting the exhaust and intake passages 191 and 181 of the engine 1, and an internal EGR system to to allow part of the burned gas in the cylinder 11 to remain substantially in the cylinder 11 .

The motor controller 100 is a controller that includes a prior art microcomputer as a basic element. The motor controller 100 includes a CPU (main processor) that executes programs, a memory configured as RAM or ROM, for example, that stores programs and data, and an I/O bus that inputs and outputs electric signals.

The engine controller 100 receives at least a signal indicative of the flow rate of intake air from an air flow meter 51, a crank angle pulse signal from a crank angle sensor 52, an accelerator pedal opening signal from an accelerator pedal opening sensor 53 which detects the degree of depression of the accelerator pedal, a vehicle speed signal from a vehicle speed sensor 54, the engine 1 cooling water temperature signal from a water temperature sensor 58, an intake temperature signal from an intake temperature sensor 59, an intake pressure signal from an intake pressure sensor 510, and an oxygen concentration in exhaust gas signal from a linear oxygen sensor 511. Based on these input signals the engine controller 100 calculates parameters for controlling the engine 1, such as a target throttle opening signal, fuel injection pulse, ignition assist signal, and valve phase angle signal. Then the engine controller 100 gives these signals to the throttle valve 55 (specifically, a throttle actuator operating the throttle valve 55), the VVT mechanism 23, 24, the fuel delivery system 57, the ignition assist assist system 56, the EGR system 510, and so on out of.

The geometric compression ratio ε of the engine 1 should be greater than or equal to 15, as described above. The geometric compression ratio may be less than or equal to 40, and is suitably in the range 20 to 35. The engine 1 is arranged to have an expansion ratio that increases as the compression ratio increases. Thus, the engine 1 has a high compression ratio and at the same time a comparatively high expansion ratio. A high geometric compression ratio stabilizes CAI combustion.

The combustion chamber 17 is formed by the inner wall surface of the cylinder 11, the top 160 of the piston 16, the bottom (i.e., the top surface 170) of the cylinder head 13, and the valve head surfaces of the intake valve 21 and the exhaust valve 22, respectively. Heat shield layers are respectively provided on the above faces to reduce cooling loss. This isolates the combustion chamber 17 from heat. The heat shield layers may be provided on all or part of the defining surfaces. Further, the heat shielding layer may also be provided on the wall surfaces of the intake port 18 and the exhaust port 19 near the openings on the ceiling portion 170 of the combustion chamber 17, although such wall surfaces are not the wall surfaces that define the combustion chamber 17 directly.

These heat shield layers are designed to have lower thermal conductivity than the base metal member forming the combustion chamber 17 to substantially prevent the heat of the combustion gas in the combustion chamber 17 from being dissipated through the defining surfaces.

The heat shield layers suitably have a lower specific volumetric heat than the base members to reduce cooling loss. In other words, it is therefore advantageous to reduce the thermal capacity of the heat shield layers so that the temperature of the surfaces defining the combustion chamber 17 changes according to the change in gas temperature in the combustion chamber 17 .

The heat shield layers can be formed by coating a base member with a ceramic material such as ZrO ₂ by plasma spray coating. The ceramic material can have a large number of pores. This allows the thermal conductivity and volumetric specific heat of the heat shield layers to be further reduced.

According to these embodiments, not only adopting the heat shield structure for the combustion chamber but also forming a heat-insulating gas layer in the combustion chamber 17 while the engine 1 is warm (that is, at a temperature greater than or equal to a predetermined temperature) allows the cooling loss to be significantly reduced can.

Specifically, injecting the fuel through the tip end of the injection nozzle of the fuel injection valve 6 into the cavity 163 on or after the compression stroke such that a Gas layer including fresh air is formed in a peripheral area inside the combustion chamber 17 and an air-fuel mixture layer is formed in the central area of the chamber, enables implementation of a layered structure in which the air-fuel mixture layer is formed in the central area inside the cavity 163 in is formed in the vicinity of the fuel injection valve 6 and a heat-insulating gas layer including fresh air is formed to cover the air-fuel mixture layer as in FIG 2 shown to surround. Here, when referring to the fuel/air mixture layer, the fuel/air mixture layer may be defined as a layer comprising a combustible fuel/air mixture, which is a fuel/air mixture with, for example, a fuel/air equivalent ratio φ is 0.1 or more. The longer the time that has elapsed since the start of fuel injection, the more the spray of fuel spreads. Thus, the size of the fuel/air mixture layer is the size measured at the time of ignition. A fuel can be considered ignited when, for example, the fuel mass ratio of the fuel becomes greater than or equal to 1%. The fuel/air mixture ignites near compression top dead center.

The heat-insulating gas layer may consist mainly of fresh air, or may additionally include a burnt gas (ie, an EGR gas) and fresh air. There is no problem even if the heat-insulating gas layer contains a small amount of the fuel. Rather, the heat-insulating gas layer needs to be leaner than the air-fuel mixture layer to function as a heat-insulating layer. As will be described later, when the load on the engine 1 increases to increase the amount of injected fuel, the heat-insulating gas layer will be a layer including a small amount of the fuel but will still be fuel-lean than it is the fuel/air mixture layer.

When the fuel-air mixture generates CAI combustion in a state where a heat-insulating gas layer and a fuel-air mixture layer as in 2 have been formed, the heat-insulating gas layer interposed between the air-fuel mixture layer and the wall surface of the combustion chamber 17 not only substantially prevents the flame of the air-fuel mixture layer from contacting the wall surface of the combustion chamber 17, but also functions as a heat insulating layer to substantially prevent heat from being transmitted from inside the combustion chamber 17 to the wall surface of the combustion chamber 17 .

It should be noted that merely reducing the cooling loss may allow the cooling loss decrease to be converted into an exhaust gas loss, which does not particularly contribute to the improvement of the specified thermal efficiency. However, this engine 1 efficiently converts the energy of the combustion gas into mechanical work corresponding to the decrease in the cooling loss by increasing the expansion ratio accompanying the increase in the compression ratio. In other words, the engine 1 remarkably improves the specified thermal efficiency by performing a configuration to reduce both the cooling loss and the exhaust gas loss.

In order to form such an air-fuel mixture layer and such a heat-insulating gas layer in the combustion chamber 17, the gas flow within the combustion chamber 17 is suitably weak at the time of fuel injection. To this end, the intake openings are arranged to have a straight shape, which either prevents or significantly reduces the creation of a vortex within the combustion chamber 17 and to keep the tumble flow as weak as possible.

(Details of Fuel Injection Control)

4 12 illustrates an example operation map of the engine 1. This operation map is divided into a light to half load range A, a half to full load range B, and a full load range C in the load direction. The light-to-full load range A, the light-to-full load range B, and the full-load range C differ mainly in the injection mode.

The light to half load range A is a range where the load on the engine 1 is the weakest. Since the load is the weakest, the amount of fuel injected becomes the smallest. In this light to half load range A, CAI combustion is performed with an air-fuel mixture layer and a heat-insulating gas layer containing substantially no fuel formed in the combustion chamber 17 as described above. Although not shown, the fuel injection valve 6 injects the fuel into the cylinder 11 during the second half of the compression stroke. The fuel can be injected either just once or multiple times. Injecting the fuel during the combustion stroke period when the pressure and temperature in the cylinder 11 becomes substantially high substantially prevents the fuel spray injected into the cylinder from traveling longer than expected. Thus, in the central area of the combustion chamber 17 an air-fuel mixture layer can be formed, and a heat-insulating gas layer can be formed around the air-fuel mixture layer. The air-fuel mixture layer spontaneously ignites in the vicinity of compression top dead center. It should be noted that in this light to half load range A, part of the burned gas is introduced into the cylinder 11 through the EGR system 512 .

The full load range C is a range where the load on the engine 1 is the heaviest. Since the load is the strongest, the amount of fuel injected becomes the largest. Allowing the fuel to ignite spontaneously in the full load range C would allow the pressure to rise so steeply during combustion that the combustion noise level increases. Thus, in the full load range C, the ignition timing is retarded up to a predetermined timing during the power stroke, causing the ignition timing to overlap with a timing when the pressure rate reaches the negative maximum value during engine operation. This reduces the rate of pressure increase during combustion and lowers the combustion noise level.

However, the temperature in the cylinder 11 drops as the power stroke progresses. Thus, retarding the ignition timing too much would cause an accidental fire. In particular, this engine 1 has such a high geometric compression ratio that the temperature in the cylinder 11 drops rapidly during the power stroke. Therefore, it is difficult to retard the ignition timing to a point where the combustion noise level can be sufficiently reduced.

Thus, in this full-load range C, the engine carries out two fuel injections, namely a pre-injection 71 and a main injection 72 after the pre-injection 71, i.e. it carries out discrete injections, as is shown in 5 is shown. By having the fuel injected into the cylinder 11 by the pilot injection 71 causing a partial oxidation reaction, a constant temperature in the cylinder 11 can be maintained even at or after the compression top dead center, allowing the fuel injected by the main injection 72 to spontaneously during or after to ignite the compression top dead center. The pilot injection is performed to raise the internal temperature of the cylinder, while the main injection 72 is performed to induce spontaneous ignition and combustion during the power stroke.

5 shows an injection mode in the full load range C. The drawing above 6 FIG. 12 illustrates sprays from a fuel mist formed in the combustion chamber 17 when the pilot injection 71 is performed in the high-load range C, and the lower diagram of FIG 6 shows the temperature distribution in the combustion chamber 17 in such a situation. The drawing above 7 FIG. 12 schematically illustrates sprays of fuel formed in the combustion chamber 17 at the compression top dead center in the high-load range C, and the lower diagram 7 shows the distribution of temperatures in the combustion chamber 17 in such a situation. The drawing above 8th FIG. 12 schematically illustrates a fuel spray formed in the combustion chamber 17 when the main injection 17 is completed in the high-load range, and the lower diagram 8th shows the temperature distribution in the combustion chamber 17 when main combustion takes place.

In the full load range C, the engine controller 100 stops the reverse flow of the EGR gas. The engine controller 100 stops the reverse flow of the EGR gas because the required load in the full load range C is so heavy that an air volume corresponding to the full load is required.

In addition, the engine controller 100 performs control such that the excess air factor λ becomes greater than or equal to one in the full load range C. The fuel injection mode in the full load range will now be described in detail.

The pilot injection 71 is performed before the middle stage of the compression stroke. Specifically, the pilot injection 71 is performed after the intake valve 21 of the engine 1 is closed during the compression stroke. The pilot injection 71 can, for example, take place at such a point in time that allows the pilot injection to end at 120-190 degrees before compression top dead center.

The pre-injection 71 consists of a predetermined number (e.g. two in the example in 5 ) of fuel injections 80, 80 together in which the lift amount of the fuel injection valve 6 is set larger than that of the main injection 72, and the fuel injection interval is set longer than that of the main injection 72. As the lift amount has increased, the mist of injected by the pilot injection has Fuel has a larger particle size and momentum.

In addition, the longer interval between injections reduces the negative pressure in the vicinity of the injection axis 6 of the fuel injector 6. The flow of spray generated in the combustion chamber 17 by the injection of the Fuel formed by the fuel injection valve 6 will catch up with the ambient air (or a gas containing the air). However, it is difficult for the air to get into the spray of fuel injected through the tip end of the fuel injection valve 6 into a hollow cone shape. If the interval between injections is narrow, the recovery of the negative pressure in the vicinity of the injection axis S slows down, and therefore the negative pressure increases. On the other hand, when the interval between injections is long, the recovery of the negative pressure in the vicinity of the injection axis S is accelerated, and therefore the negative pressure decreases.

The spray of fuel injected by the pilot injector 71 has a larger particle size, larger momentum, and less negative pressure. Thus, the spray is not easily drawn toward the negative pressure. As a result, with respect to the injection axis S of the fuel injection valve 6 as its central axis, the fuel spray spreads radially outward in a large area and travels a relatively long distance. At a point of time when the fuel spray injected by the pilot injection 71 causes partial oxidation, the fuel spray has reached a radial (that is, in the radial direction of the cylinder 11) peripheral portion 17b of the combustion chamber 17 . Here, the "radial peripheral area of a cylinder" means the outer area when the cylinder is radially divided into two areas at half the maximum diameter, such as in 6 shown. The “radial middle area of the cylinder”, on the other hand, designates the inner area when the cylinder is divided radially into two areas at half the maximum diameter. In other words, the pilot injection 71 is performed at such a timing that allows the fuel spray to have reached the radially peripheral portion 17 when the spray causes an oxidation reaction. As a result, at a time when the spray of fuel injected by the pilot injection 71 causes an oxidation reaction, the concentration of the fuel in the radially peripheral portion 17b of the combustion chamber 17 becomes higher than that of the fuel in the radially central portion 17a thereof.

In this case, the pilot injection 71 is performed after the intake valve 21 has been closed as described above. Thus, the flow of intake air has already leveled off to some extent, and therefore the flow of fuel that has reached the radial peripheral portion 17b of the combustion chamber 17 tends to remain there.

The amount of fuel injected by the pilot injection 71 is set to be an amount defined by such an air-fuel ratio at which the fuel causes a partial oxidation reaction. The air-fuel ratio at which the fuel causes a partial oxidation reaction may be an air-fuel ratio at which the excess air factor λ becomes 8 or more. Setting the excess air factor λ to be greater than or equal to 8 reduces the reaction at which CO in the fuel is allowed to be converted into CO ₂ . That is to say, the fuel produces an oxidation reaction, but produces an insufficient thermal flame reaction.

Meanwhile, the main injection 72 may be performed to start at a timing before the compression top dead center and end at a timing after the compression top dead center. In particular, the injection timing of the main injector 72 may be set to be such a timing that allows the fuel to ignite at a predetermined retarded period of the power stroke. Here, the “predetermined delayed period” means a combustion period that overlaps with a point in time when the rate of pressure increase in the cylinder 11 reaches a negative maximum value during operation of the engine.

The main injection 72 has a predetermined number (e.g. six in the in 5 illustrated example) of fuel injections 90, 90...., in which the lift amount of the fuel injection valve 6 is set to be smaller than that of the pilot injection 71 and in which the fuel injection interval is set to be shorter than that of the pilot injection 71. This means that the spray of fuel injected by the main injector 72 has a smaller particle size and a smaller impulse. Further, since the interval between fuel injections is narrow, the recovery of the negative pressure in the vicinity of the injection axis S of the fuel injection valve 6 is slowed down as described above, whereby the negative pressure in the vicinity of the injection axis S increases. The spray of the fuel injected by the main injection 72 has a smaller particle size and a smaller impulse, and is thereby influenced by the negative pressure so much that it spreads less radially outward with the injection axis S as the central axis and only travels a relatively short distance . Consequently, the fuel spray injected by the main injector 72 tends to stay in the radially central portion 17a of the combustion chamber 17, as in FIG upper drawing of 7 is illustrated. As a result, at a time when the fuel spray injected by the main injection 72 ignites, the concentration of the fuel in the radially central portion 17a of the combustion chamber 17 becomes higher than that of the fuel in the radially peripheral portion 17b thereof.

The main injection 72 is an injection to generate main combustion (that is, combustion that generates the largest amount of heat in one stroke) that generates engine torque. Thus, their fuel amount is set to be an appropriately determined amount with respect to the required torque. For example, in the case of the main injection 72, at least ¾ of the total fuel quantity is injected, which is the sum of the fuel injected by the pilot injection 71 and the fuel injected by the main injection 72.

First, how to combust by the pilot injection 71 and the main injection 72 described above will be described.

The pilot injection 71 is performed during the compression stroke as indicated by the upper drawing in FIG 6 shown. The spray of the fuel injected by the pilot injection 71 moves toward the radial peripheral portion 17b of the combustion chamber 17 as described above. At this time, the temperature in the combustor 17 is a radially uniform predetermined temperature (e.g., 1000K) as in the lower graph of FIG 6 is shown.

Thereafter, as the compression stroke progresses, the temperature of the air-fuel mixture including the fuel spray injected by the pilot injector 71 increases with compression.

Following this, the main injection 72 begins at a predetermined point in time before compression top dead center. As described above, the spray of fuel injected by the pilot injection 72 is concentrated toward the radially central portion 17a of the combustion chamber 17 (see the upper drawing of FIG 7 ).

Meanwhile, the spray of the fuel injected by the pilot injection 71 remains in the radial peripheral portion 17b. Once the in-cylinder temperature rises with the compression stroke, the fuel spray will soon cause an oxidation reaction. This oxidation reaction can be caused, for example, in the vicinity of the compression top dead center. However, since this oxidation reaction is a partial oxidation reaction, the temperature rise caused by the oxidation reaction is not so remarkable. Further, this partial oxidation reaction is mainly caused in the radial peripheral portion 17 b of the combustion chamber 17 . Therefore, the in-cylinder temperature rises sharply in the radial peripheral portion 17b as shown in the lower graph 7 is shown. The temperature in the radial peripheral area 17b can rise by several hundred Kelvin K, for example. The two-dot-dash line in 7 indicates the in-cylinder temperature when the pilot injection 71 is performed. In this case, the air-fuel mixture in the radially peripheral portion 17b expands by the oxidation reaction, and therefore the air-fuel mixture or air in the radially central portion 17a undergoes adiabatic compression and thus becomes elevated in temperature. The temperature in the radially central portion 17a may rise by several tens of K, for example.

It should be noted that by stopping the reverse flow of the EGR gas in the high-load range, the environment inside the combustion chamber 17 is in a state where the spray of fuel injected by the pilot injector 71 tends to be oxidized relatively easily.

Meanwhile, the main injection 72 continues while the fuel injected by the pilot injection 71 causes the oxidation reaction, and ends at a predetermined timing after the compression top dead center. The spray of the fuel injected by the pilot injection 72 concentrates toward the radially central portion 17a of the combustion chamber 17 shown in the upper drawing 8th implied. The temperature in the radially central portion 17a has risen by the partial oxidation reaction of the fuel in the radially peripheral portion 17b as described above. Thus, the temperature of the radially central portion 17a is kept at a temperature at which the fuel can spontaneously ignite for a while even after entering the power stroke. As a result, the fuel spray injected by the main injector 72 will spontaneously ignite at a predetermined ignition timing delay after the injection has ended. This produces the main combustion, causing the temperature in the radially central portion 17a to rise sharply as shown in the drawing below 8th can be seen. In 8th the two-dot chain line indicates the in-cylinder temperature when the fuel injected by the pilot injection 71 causes an oxidation reaction.

Normally, with a delayed main burn, there is a time limit on the delay. That means that as soon as the As the intake stroke progresses, the in-cylinder temperature drops with an increase in volume of the combustion chamber 17 . Therefore, excessively delaying the main combustion would cause an accidental fire. The higher the compression ratio, the higher the rate of decrease in in-cylinder temperature during the intake stroke. Thus, the higher the compression ratio, the shorter the allowable length of deceleration. However, maintaining the temperature in the radially central region 17a where the fuel spray injected by the main injection 72 disperses makes it possible, by the pilot injection 71, to lengthen the allowable length of the main combustion even after the compression top dead center.

However, merely raising the in-cylinder temperature at or after compression top dead center would allow the fuel injected by the main injector 72 to ignite locally before mixing with the air in the combustion chamber 17, possibly producing soot. However, inducing the fuel spray injected by the pilot injection 71 the oxidation reaction in a region other than where the fuel spray injected by the main injection 72 disperses can prevent an excessive temperature rise in the region where the fuel spray injected by the main injection 72 disperses becomes. As a result, this substantially prevents the fuel injected through the main injector 72 from causing localized sooting.

The combustion period of the main combustion thus delayed (that is, a period in which the fuel has a combustion mass ratio of 10 - 90%) overlaps with a point in time when the pressure increase rate during the operation of the engine reaches a negative maximum value. Alternatively, the center of gravity of the main combustion thus delayed overlaps with a period in which the combustion chamber has a significant negative pressure rise rate (for example, 10 to 20 degrees after compression top dead center). This means that the delayed spontaneous ignition and combustion takes place in a period where the pressure rise rate is inherently low. For this reason, the maximum value of the pressure increase rate when the combustion is realized as the delayed spontaneous ignition and combustion has remarkably decreased compared to the maximum value of the pressure increase rate when the combustion is realized as the normal spontaneous ignition and combustion. As a result, the combustion noise level (i.e., part of the noise, vibration and crackle (NVH)) of delayed spontaneous ignition and combustion is significantly reduced compared to the combustion noise level of normal spontaneous ignition and combustion.

It should be noted that in order to decrease the pressure rise rate during combustion, the EGR amount needs to be increased. However, increasing the amount of EGR would result in a reduced amount of fresh air. As a result, sufficient torque could not be secured. The magnitude of the pressure increase rate becomes a problem particularly in a full load operation range where significant torque is required. On the other hand, the above configuration ensures a required amount of fresh air, thereby ensuring the generation of sufficient torque.

Referring back to the in 4 As shown in the operation map, the half to full load range B is a range where the load on the engine 1 is between the light to full load range A and the full load C range. In the half to full load range B, a smaller amount of fuel is injected than in the full load range C. If the fuel is allowed to spontaneously ignite in the half to full load range B, there is no need for retarded ignition timing during the power stroke, unlike in the full load range C In the half to full load range B, a larger amount of fuel is injected than in the light to half load range A. As a result, it facilitates the generation of smoke in the half to full load range B compared to the light to full load range A. It is thus recommended substantially to prevent the fuel concentration from rising too much by effectively using the air in the combustion chamber.

Thus, in the medium to heavy load range B, the above-described wide distribution of the fuel spray in the combustion chamber while maintaining the adiabatic function of the gas layer above a certain extent substantially prevents smoke generation. In particular, like this in 9 As shown, the fuel is injected twice by the pilot injection 71 and the main injection 72. 9 shows a fuel injection mode in the half to full load range B. 10 12 schematically represents a fuel/air mixture layer formed in the combustion chamber 17 in the half to full load range B.

In the half to full load range B, the engine controller 100 causes the EGR system 512 to flow the EGR gas from the exhaust gas passage 191 back into the intake passage 181 . In this half to full load range B, the EGR gas to flow back is an EGR gas that has been cooled by an EGR cooler.

The engine controller 100 performs control such that the excess air factor λ becomes almost equal to 1 (ie, λ≈1) by reverse flow of the EGR gas in the half to full load range B. Adjusting the excess air factor λ enables a three-way catalyst to perform the exhaust gas purification function. Next, the pilot injection 71 and the main injection 72 in the half to full load range B will be described in detail.

The pilot injection 71 in the half to full load range B is performed before the middle stage of the compression stroke. Specifically, the pilot injection 71 is performed after the intake valve 21 of the engine 1 is closed during the compression stroke. For example, pilot injection 71 may be performed at a timing that allows fuel injection to end at 120 to 90 degrees before compression top dead center. Alternatively, the pre-injection 71 can also be carried out over a period of time that extends from the intake stroke to the first half of the compression stroke.

The pilot injection 71 is performed at a relatively large lift amount. The spray of fuel injected by the pilot injector 71 has a larger particle size and momentum. Thus, the spray of fuel injected by the pilot injector 71 travels a relatively long distance.

Further, since the pilot injection 71 is performed before the main injection 72, the spray of the fuel injected by the pilot injection 71 reaches the radial (that is, in the radial direction of the cylinder 11) peripheral portion 17b of the combustion chamber 17 when the main injection 72 is performed (see Fig. 10 ). In the radial peripheral area 17b of the combustion chamber 17, a narrow gap 17c (hereinafter referred to as “squish area”) remains between a peripheral portion of the piston 16 and the ceiling portion of the cylinder 11. At least at the time of ignition, the fuel spray injected by the pilot injection 71 has the squish area 17c reached. In other words, the pilot injection 71 is performed at such a timing as to allow its fuel spray to reach the squish area 17c at the timing of ignition.

In this case, when the pilot injection 71 is performed after the intake valve 21 is closed as described above, the intake air flow has already settled to some extent, and therefore the fuel spray that has reached the radial peripheral portion 17b of the combustion chamber 17 tends , to stay there.

It should be noted that the amount of fuel injected by the pilot injection is smaller than that injected by the main injection 72 .

The main injection 72 may be performed at a timing that is such as to allow the injection to be completed before the compression top dead center and after the pilot injection 71 . The main injection 72 may be performed in a period lasting from the final stage of the compression stroke to the initial stage of the power stroke. The main injection 72 is a multi-stage injection that includes a plurality of fuel injection events.

In particular, the main injection 72 comprises a first group of injections 8 and a second group of injections 9. The first group of injections 8 is composed of a predetermined number (e.g. two in the in 9 illustrated example) of fuel injections 80, 80 in which the lift amount of the fuel injection valve 6 is set to be larger than that of the second group of injections 9 and the fuel injection interval is set to be longer than that of the second group of injections 9 The second group of injections 9 has a predetermined number (e.g. four in the in 9 illustrated example) of fuel injections 90, 90... in which the lift amount of the fuel injection valve 6 is set to be smaller than that of the first group of injections 8 and the fuel injection interval is set to be shorter than that of the first group of injections 8. The lift amount of the first group of injections 8 is approximately the same as that of the pilot injection 71. The lift amount of the second group of injections 9 is smaller than that of the pilot injection 71. It should be noted that the in 9 Fuel injectors 80 shown do not necessarily have the same displacement as those in FIG 5 Fuel injections 80 shown 9 Fuel injectors 90 shown do not necessarily have the same displacement as those in FIG 5 shown fuel injections 90.

The first group of fuel injections 8 forms a spray of the fuel with a larger particle size and has a wide negative pressure region as described above and therefore forms in the direction of travel a spray of the relatively long-distance traveling and fuel spread far in the radial direction. On the other hand, the second group of injections 9 forms a spray of the fuel with a smaller particle size and has a shrunk negative pressure area, and therefore forms a spray of the fuel advancing in the traveling direction and narrowly spread in the radial direction.

As a result of these fuel injections by the first and second groups of injectors 8 and 9, at the time of ignition (that is, at a predetermined time after compression top dead center), the fuel spray has already spread widely to some extent in the radial direction within the combustion chamber 17.

In particular, a fuel spray with a relatively large particle size (that is, with a larger pulse) is injected by the pre-injection 71 at a relatively early point in time. Thus, at least at a point in time when the pilot injection 72 is performed, an air-fuel mixture is formed in which the fuel is more concentrated in the radially peripheral area 17b than in the radially central area 17a in the combustion chamber 17. In this case, it enables conducting the pilot injection 71 as a single-stage injection allows the fuel spray to travel a longer distance compared to conducting the pilot injection 71 as multi-stage injections.

Thereafter, a fuel spray having a relatively small particle size (i.e., having a smaller impulse) is injected at a relatively late timing through the main injector 72 to form an extended vacuum region. Thus, at least at a point in time of ignition, a fuel/air mixture is formed in the combustion chamber 17, in which the fuel has a higher concentration in the radial central area 17a than in the radial peripheral area 17b. In this case, the combination of the first group of injections 8 in which the lift amount of the fuel injection valve 6 is set to be relatively large and the fuel injection interval in the second group of injections 9 in which the lift amount is set to be relatively small be, in the direction of travel, a fuel spray of fuel that travels a relatively long distance in the direction of travel and spreads widely in the radial direction, and a spray of fuel that travels a relatively short distance in the direction of travel and spreads narrowly in the radial direction, which it allows the fuel spray to spread widely in the radially central portion 17 a of the combustion chamber 17 .

It should be noted that the spray of fuel injected by the first group of injectors 8 only travels a longer distance and spreads further radially than the spray of fuel injected by the second group of injectors 9 . Thus, the spray of fuel injected by the first group of injectors 8 travels a shorter distance and spreads less radially than the spray of fuel injected by the pilot injector 71 .

As described above, the pilot injection 71 has the spread fuel spray in the radial peripheral portion 17b of the combustion chamber 17 in a larger amount than in the radially central portion 17a thereof. On the other hand, the main injection 72 has the fuel spray in a larger amount in the radially central portion 17a of the combustion chamber 17 than in the radially peripheral portion 17b thereof. These pre-injections and main injections 71, 72 form a fuel/air mixture which is distributed further radially in the combustion chamber 17 than at the time of ignition. That is to say, these pilot and main injections 71, 72 substantially prevent a locally higher fuel concentration section from being formed.

Then, when the pilot injection 71 and the main injection 72 are completed, the fuel will ignite. This means that the fuel spray ignites after a fuel/air mixture has been formed that is intended to be widely distributed radially in the combustion chamber 17 . Thus, this essentially prevents smoke from being produced.

In this case, the pilot injection 71 is performed relatively early, but the amount of fuel injected by the pilot injection 71 is smaller than the amount of fuel injected by the main injection 72 and is not so large, enabling prevention of premature ignition. Further, in this operation range, the EGR gas is present as an inert gas in the combustion chamber 17 . This also essentially makes it possible to avoid the pre-injection 71 igniting the spray prematurely.

(Fuel injection control in the half to full load range)

In the half to full load range B, the fuel is injected almost twice, as described above, by the pilot injection 71 and the main injection 72. In this case, depending on the pressure condition in the cylinder 11, the temperature condition in the cylinder 11 and the oxygen concentration in the cylinder 11 injected into the cylinder 11 by the pilot injection 71 Fuel cause a partial oxidation reaction during the second half of the compression stroke. Once the partial oxidation reaction has been induced, the temperature in the cylinder 11 rises accordingly. Thus, the fuel injected by the main injector 72 will spontaneously ignite before it has been given sufficient ignition timing retard, thereby either producing smoke or extending the combustion period during the power stroke to increase CO emissions.

11 12 shows an example variation in the heat generation rate in the cylinder 11 in the half to full load range B. 12 shows an exemplary variation of the average temperature in the half to full load range B. The dashed characteristic curves in 11 and 12 show a situation where the fuel was injected twice, by the pilot injection 71 and the main injection 72, and show an example where the fuel injected by the pilot injection 71 causes a partial oxidation reaction during the second half of the compression stroke. As in 11 shown, the generation of the partial oxidation reaction results in the generation of heat, albeit a small amount. Furthermore, as in 12 shown, the temperature in the cylinder 11 also indicates.

Performing the main injection 72 in such a state allows the fuel injected by the main injection 72 to spontaneously ignite in the vicinity of the compression top dead center without being given a sufficient ignition timing retard. Insufficient mixing of fuel and air leads to the formation of smoke. Furthermore, the insufficient mixing of the fuel and the air allows the heat to be generated continuously even during the latter half of the combustion, thus causing an increase in CO emissions.

Thus, in this engine 1, the engine controller 100 determines whether a partial oxidation reaction will occur during the second half of the compression stroke. If the answer is YES, the engine controller 100 performs an injection through the phantom line in between the pilot injection 71 and the main injection 72 9 indicated intermediate injection through. The vaporization latent heat of the fuel spray injected into the cylinder 11 by the intermediate injection causes a temperature drop in the cylinder 11, thus preventing the partial oxidation reaction from occurring.

The engine controller 100 estimates the temperature state in the cylinder 11 based on the cooling water temperature detected by the water temperature sensor 58 and the intake air temperature detected by the intake air sensor 59 Cylinder 11. The engine controller 100 further estimates the oxygen concentration in the cylinder 11 based on the oxygen concentration in the exhaust gas detected by the linear oxygen sensor 511, and thus determines whether there is a partial oxidation reaction during the second half of the compression stroke will come. For example, the engine controller 100 may determine in advance whether or not a partial oxidation reaction will occur based on a predetermined model and the particular parameters described above. Alternatively, the engine controller 100 may later determine that the partial oxidation reaction has occurred based on the constituents of the exhaust gas.

The intermediate injection 73 is performed during a predetermined period between the pilot injection 71 and the main injection 72 as indicated by the dotted lines in FIG 9 is indicated. The intermediate injection 73 is a single-stage fuel injection. The intermediate injection 73 is performed before the partial oxidation reaction occurs during the second half of the compression stroke. As described above, the intermediate injection 73 is suitably performed before the partial oxidation reaction occurs in order to prevent the partial oxidation reaction from occurring by lowering the temperature in the cylinder 11 with the latent heat of vaporization of the injected fuel.

Performing the intermediate injection 73 prematurely would lower the temperature in the vaporization latent heat cylinder 11 before a period at which the partial oxidation reaction will occur. In this case, the function of preventing the partial oxidation reaction from occurring would not be sufficiently performed. Further, the fuel injected by the intermediate injection would also be exposed to the high-temperature/high-pressure environment in the cylinder 11, possibly causing the partial oxidation reaction during the second half of the compression stroke. Thus, the intermediate injection 73 is suitably performed at a timing that allows the fuel injected by the intermediate injection 73 to spontaneously ignite during or after the ignition of the fuel injected by the main injection 72 .

This gives the intermediate injection 73 a relatively long interval between itself 73 and the main injection 72 that follows it. Concretely, the interval between the end of injection of the intermediate injection 73 and the start of the main injection 72 is longer than the interval between first and second injections of the main injection 72 which is multi-stage injection. This substantially prevents the spray of fuel injected by the intermediate injection 73 from interfering with the spray of fuel injected by the main injection 72 . This advantageously helps reduce cooling loss by lowering combustion temperature and substantially preventing smoke generation by eliminating local formation of an overly dense fuel/air mixture.

The amount of fuel injected by the intermediate injection 73 is smaller than that of fuel injected by the main injection 72 . Since the intermediate injection 73 starts earlier than the main injection 72, the temperature and pressure in the cylinder 11 at the time of the intermediate injection 73 are not yet sufficiently high. Thus, the fuel spray injected into the cylinder 11 becomes more and more likely to travel longer than expected and possibly come into contact with the wall surface of the combustion chamber 17 . This is disadvantageous when forming the gas layer surrounding the fuel/air mixture. For this reason, the amount of fuel injected by the intermediate injector 73 is relatively small. Further, reducing the fuel injected by the intermediate injector 73 enables the injection of much fuel by the main injector 72. This favors the engine torque in the half to full load range B where the total amount of fuel injected increases.

In the 11 and 12 the solid line indicates an exemplary deviation of the heat generation rate and an exemplary deviation of the average temperature in the cylinder 11, each in a situation where the pilot injection 71, the intermediate injection 73 and the main injection 72 are performed. As described above, performing the intermediate injection 73 at a predetermined timing allows the temperature in the cylinder 11 to be as shown in FIG 12 shown is lowered, thus essentially preventing the partial oxidation reaction from occurring (cf. 11 ). The fuel injected by the main injector 72 is allowed to ignite spontaneously and without allowing sufficient ignition timing retard. In 11 the peak of heat generation accompanying the spontaneous ignition as indicated by the solid line is delayed with respect to the peak of heat generation as indicated by the broken line. Also, the combustion ends quickly during the second half of the combustion. This makes it possible to reduce CO emissions. In this way, the partial oxidation reaction in the full load range C is intentionally utilized, whereas the partial oxidation reaction in the half to full load range B is substantially prevented from occurring.

It should be noted that in the 5 and 9 In the examples shown, the pilot injection 71 and the main injection 72 are each implemented as a multi-stage injection, which includes a plurality of fuel injections. Alternatively, the fuel injections may be continuously performed while keeping the lift amount of the fuel injection valve 6 constant for a predetermined time. Even in this case, the fuel spray can also be formed in the same manner as in the multi-stage injections.

In the above-described example, as the fuel injection valve 6, an outwardly opening fuel injection valve 6 is used. For example, a VCO (Valve Covered Orifice) type injector can also change the effective cross-sectional area of the injector nozzle orifice by adjusting the degree of cavity created by the nozzle orifice. Consequently, even if the in the 5 or 9 illustrated fuel injection mode is applied, an air-fuel mixture layer can be formed in the central portion of the cavity 163, a heat-insulating gas layer can be formed around the air-fuel mixture layer, and an overly dense air-fuel mixture can be formed in the same manner as in the outward opening fuel injection valve are substantially prevented from forming locally.

In the example described above, a heat shielding structure is used for the combustion chamber and the intake port, and a heat insulating gas layer is formed inside the combustion chamber. However, the present disclosure is also applicable to an engine that does not employ such a heat shield structure.

reference list

1

engine

100

Engine controller (fuel injection control unit)

11

cylinder

12

cylinder block

13

cylinder head

16

Pistons

17

combustion chamber

6

fuel injector

Claims (4)

A fuel injection control device for a direct injection engine, the control device comprising: an engine block including a combustion chamber defined by a cylinder head cover portion, a cylinder block provided for a cylinder, and a piston reciprocating inside the cylinder; and a fuel injection control unit including a fuel injection valve provided within the combustion chamber to inject a liquid fuel and configured to inject the fuel into the combustion chamber at a predetermined timing, wherein the fuel injection control unit is adapted to perform a main injection over a period ranging from a final stage of a compression stroke to an initial stage of a power stroke and also a pilot injection over a period lasting from the intake stroke to a first half of the compression stroke, to be carried out in which a smaller quantity of fuel is injected than in the case of the main injection, and the fuel injection control unit is also configured to determine whether the fuel injected by the pilot injection causes a partial oxidation reaction during a second half of the compression stroke, and when determining that the fuel causes the partial oxidation reaction, an intermediate injection between the pilot injection and the main injection to perform, and the fuel injection control unit is adapted to perform the intermediate injection at such a timing that allows the fuel injected by the intermediate injection to ignite spontaneously during or after the fuel injected by the main injection spontaneously ignites, said Time before the partial oxidation reaction occurs during the second half of the compression stroke. Fuel injection control device for the direct injection engine claim 1 wherein the fuel injection control unit is configured to determine whether or not a partial oxidation reaction will occur based on an in-cylinder pressure, an in-cylinder temperature, and an in-cylinder oxygen concentration. Fuel injection control device for the direct injection engine claim 1 or 2 , where the intermediate injection injects a smaller amount of fuel than the main injection. Fuel injection control device for the direct injection engine according to any one of Claims 1 until 3 wherein the main injection is a multi-stage injection including a plurality of fuel injections, an interval between an injection end of the intermediate injection and an injection start of the main injection is longer than an interval between first and second injections of the main injection.

Images